assume cs:codesg
codesg segment

mov ax,4c00h
int 21h

start:
mov ax,0

s:
nop
nop

mov di,offset s
mov si,offset s2
mov ax,cs:[si]
mov cs:[di],ax

s0:
jmp short s

s1:
mov ax,0
int 21h
mov ax,0

s2:
jmp short s1
nop

codesg ends

end start